It is desirable to use starch, which is a vegetable carbohydrate, as a natural plastic material, in various areas employing known methods of plastics processing. Owing to their granular structure, however, natural starches must be destructurized before they can be thermoplastically processed.
U.S. Pat. No. 4,673,438 describes a six-stage injection molding process for producing a shaped article from a starch-water composition having a water content of 5% to 30% by weight. A disadvantage of the high water content is that a special apparatus is required so that the plasticizer water does not escape in the form of steam at the high processing temperatures.
WO-OS 90/05 161 describes a process for producing thermoplastically processible starch, wherein the reduction of the melt temperature of the starch or starch derivatives is achieved by addition of at least 5% to 35% by weight of additives having a defined solubility parameter. WO-OS 90/14 938 provides a process for producing a shaped article from water and starch material having a high amylose content; the process includes degasification.
The methods of both of the above-mentioned WO Applications have certain disadvantages. When employing processes which are normal in the plastics industry in which the additives (for example plasticizers) are introduced into the polymer melt in the solid state downstream of the feed zone, inhomogeneities in the starch melt and the granulates derived therefrom are produced. Thus, the remainder of the processing operation is impaired as, for example, by strand breakages. Moreover, it is not possible to produce a homogeneous, thermoplastically processible starch melt with apparatus customary in the plastics industry and designed for the processing of polymers. The prior art lacks the necessary teaching for this.
Furthermore, the usual method of controlling the melt viscosity via the plasticizer content has proven unsuitable, as the necessary large quantities of plasticizers also reduce the stability of the melt so that the melt stream tears; this is particularly disadvantageous during the production of films.